From the U.S. Pat. No. 4,454,382 there is known an arrangement for releasable attachment by means of engagement members. Said engagement members include hook-like members which are intended to engage with flanges arranged on attachment rails. With this known solution, one and the same attachment arrangement can be used for different types of attachment rails. The attachment is effected by means of mould engagement, which is secured by means of a pressure spring. In this connection, considerable spring force is required for attachment of heavier units, through which an attachment with limited reliability is obtained, unless extremely substantial spring forces are applied. In for example, an environment which is exposed to vibrations, there is a risk of the unit, in some disadvantageous situations, vibrating until it comes loose.